Init scripts source an optional defaults file if it exists

This helpful addition allows services to inject optional configurations into the initialization script, following common distribution standards:
* [/etc/default/$SERVICE_NAME for Debian](https://www.debian.org/doc/debian-policy/ch-opersys.html#s-writing-init)
* [/etc/sysconfig/$SERVICE_NAME for Fedora](http://fedoraproject.org/wiki/Packaging:SysVInitScript#Initscript_packaging) and RHEL
* [/etc/defaults/$name for FreeBSD](https://www.freebsd.org/doc/handbook/configtuning-configfiles.html)